The conventional devices for effecting a resistance are generally provided with a friction means, an oil pressure means, a vortex, a generator, etc. These conventional devices are generally defective in design in that they are capable of effecting a resistance of a limited intensity, and that they are incapable of pulling back an object in motion. As a result, the application of the conventional devices is limited to the exercise machine, such as the exercise bicycle or the like. In other words, the conventional devices can not be used in conjunction with the exercise devices, such as an arm developing device, a rowing device, a chest building device, etc. the conventional exercise devices are generally provided with a lead block as a load. The lead block falls back to its original position as soon as the lead block is releved of the application force. The user of such conventional exercise devices is susceptible to bodily injury which is caused by the reaction force of the lead block. In other words, the lead block is a primitive means for providing an exercise device with a load. Among the conventional devices for effecting a resistance, the most advanced device is provided with an electromagnetic coil as the magnetic field source for effecting the resistance, which can be adjusted in magnitude by the digital or analog device, as shown in FIG. 1. This prior art device is provided with an electromagnetic core A on which an electromagnetic coil R is wound. The electromagnetic core A is of a U-shaped construction and is provided with a space M in which the rim of a braking wheel is received. When the coil is provided with the electric current, a magnetic field is brought about. In the meantime, the magnetic flux is brought about on the core A. As the braking wheel is actuated to turn, the rim of the braking wheel in motion moves through the space M of the core A, thereby resulting in the production of eddy current which forms a reverse rotation moment serving as a load to the braking wheel. Such a prior art device as described above with reference to FIG. 1 is defective in design in that the application point of the resistance effected by the device is relatively small, and that the magnetic loop of the prior art device is vulnerable to magnetic leakage. As a result, the braking wheel in motion can not be effectively controlled by the resistance which is brought about by the prior art device.